The initial step in information processing by higher organisms is the sensing of a stimulus from the environment. Once detected, the stimulus must be transduced to a signal which can be transmitted to the brain. Of the various sensory systems (taste, touch, hearing, smell, and vision), we presently know most about vision. Nevertheless, we still do not understand the series of molecular events that start with detection of light and lead to nerve excitation. We do know that the sensory stimulus, the photon, is detected in specialized photoreceptor cells by a Vitamin A derivative, 11-cis retinal which is covalently bound to the membrane protein rhodopsin. Upon absorption of light, this chromophore goes through a sequence of spectrally characterized events which result in the isomerization of the double bond from 11-cis to 11- trans. Subsequently, there occurs a change in the electrical potential of the cell's membrane. The objective of the proposed research is to determine how these molecular events trigger a bioelectrical signal which can be transmitted to the brain. The sequence of spectral events, which occur in less than 1 millisecond, will be studied by rapid (and "ultra-rapid") kinetic techniques. The kinetic studies are designed to probe the features of the chromophore site, features of the protein structure, and features of the membrane environment which are required by rhodopsin to function in the triggering process. Our chance of understanding the mechanism of visual reception and transduction as molecular events is most promising. With this understanding may come the necessary insight which will allow us to undertake the study of the molecular basis of our other sensory processes.